This invention relates to a radio transmitter for use in a mobile telephone system, a multichannel access (MCA) system, or the like to transmit an output signal in response to an input signal.
A radio transmitter of the type described comprises an amplifier section which will mainly be specified by a high frequency power amplifier which forms an output stage of the amplifier section and which is connected to an antenna, although an intermediate frequency amplifier and a low frequency amplifier are also included in the amplifier section. Specifically, the high frequency power amplifier may be considered as a combination of a variable gain high frequency power amplifier circuit and a high frequency power amplifier circuit. The variable gain high frequency amplifier circuit is supplied with an internal signal derived from an input signal to produce a gain controlled signal while the high frequency power amplifier circuit amplifies the gain controlled signal into the output signal.
In such a radio transmitter, it is necessary to stably keep an output signal at a predetermined level. In other words, an output transmission power level should be preferably held substantially constant in the radio transmitter. To this end, an automatic level control (ALC) circuit is used in combination with the high frequency power amplifier.
The automatic level control circuit comprises a detector supplied with the output signal to produce a detection signal, namely, a d.c. signal specifying a power level of the output signal. The detection signal is sent to a comparator and is compared in the comparator with a reference signal predetermined for the power level of the output signal to produce a control signal representative of a difference between the detection signal and the reference signal. The control signal is delivered to a power source control circuit which is connected to the variable gain high frequency power amplifier circuit and which is also connected to a power source. Supplied with the control signal, the power source control circuit controls the power source to supply controlled electric power to the variable gain high frequency power amplifier circuit and to thereby control a gain of the variable gain high frequency power amplifier circuit. As a result, the gain controlled signal is sent from the variable gain high frequency power amplifier circuit to the high frequency power amplifier circuit to be produced as the output signal. Thus, the output signal has an output level determined by the reference signal given to the comparator circuit.
It is to be noted here that the detector is implemented by a semiconductor or a primary diode which has a temparature characteristic and that the antenna has a frequency characteristic like in a coupling circuit between the high frequency power amplifier and the antenna. Therefore, the detection signal is varied in its amplitude in dependency upon the temparature characteristic of the primary diode. In addition, the detection signal is also varied by the frequency characteristic of the antenna or the coupling circuit when frequencies are changed from one to another. Inasmuch as the level of the output signal is determined by the detection signal, as mentioned before, the output signal is varied in dependency upon the detection signal even when the reference signal is kept constant.
Heretofore, it is known in the art to compensate for the temperature characteristic of the semiconductor or primary diode by the use of a subsidiary diode having the same temperature characteristic as the primary diode.
However, no attempt has been tried to compensate for the frequency characteristic of the antenna or the coupling circuit except that the coupling circuit is subtly adjusted.